This invention relates to resonant circuits and particularly to active means for not only varying the Q of an inductor but also increasing it far beyond that value of a Q which is normally considered as the inherent maximum of the passive components.
The figure of merit Q of a resonant circuit at a frequency f, containing inductance and capacitance is defined by the relationship Q = .omega.L/R, where .omega. = 2.pi.f, L is the circuit inductance, and R is the net circuit resistance. In most cases the capacitors used in resonant circuits are non-dissipative at low frequencies relative to other losses which may exist in the circuit, so for all practical purposes the resistance R is primarily that of the inductor windings.
As the frequency of a resonant circuit is lowered it becomes increasingly difficult to obtain the high Q values which are enjoyed at higher frequencies. This arises because the inductive reactance .omega.L decreases in direct proportion to frequency while the circuit resistance R remains unchanged or may actually rise as inductors with larger windings are used. As a result of this generally lower Q, frequency selective LC circuits perform poorly at frequencies below about 100 kH.sub.z.
According to the present invention, active circuit means are provided which dramatically reduce the problem of low Q at frequencies below about 100 kH.sub.z. The circuits utilized permit the Q of an inductor to be made variable and increased far beyond that normally considered to be the inherent maximum of the passive components. The resulting circuit improvement over the prior art should find application in such areas as receiving equipment, filter systems, and frequency selective equipment operating at low frequencies.